β2 Adrenergic receptor agonists are recognized as effective drugs for the treatment of pulmonary diseases such as asthma and chronic obstructive pulmonary disease (including chronic bronchitis and emphysema). β2 Adrenergic receptor agonists are also useful for treating pre-term labor, and are potentially useful for treating neurological disorders and cardiac disorders. Commonly assigned U.S. Pat. No. 6,576,793 B1 discloses the novel compound N-{2-[4-((R)-2-hydroxy-2-phenylethylamino)phenyl]ethyl}-(R)-2-hydroxy-2-(3-formamido-4-hydroxyphenyl)ethylamine,
as a potent β2 adrenergic receptor agonist. Compound 1 is alternatively referenced by the chemical names N-[3-[(1R)-1-hydroxy-2-[[2-[4-[((2R)-2-hydroxy-2-phenylethyl)amino]phenyl]ethyl]amino]ethyl-6-hydroxyphenyl]-formamide, (α-R)-3-formamido-4-hydroxy-α-[[[p-(N-((2R)-hydroxy-phenethyl))-amino-phenethyl]amino]methyl benzyl alcohol, and N-[2-hydroxy-5-[(1R)-1-hydroxy-2-[[2-[4-[[(2R)-2-hydroxy-2-phenylethyl]amino]phenyl]ethyl]amino]ethyl]phenyl]-formamide.
Active agents for the treatment of pulmonary diseases are advantageously administered by inhalation. Preparation of formulations for administration by inhalation typically relies on the existence of a crystalline form of the active agent, or of a crystalline form of a pharmaceutically acceptable salt of the active agent, having suitable physical and chemical properties. For example, crystalline salts used in formulations administered by inhalation typically must be non-hygroscopic and thermally stable. Thus it would be desirable to provide a crystalline form of compound 1 or of a salt thereof that is stable upon storage even under conditions of elevated temperature and relative humidity.